// This doesn't seem to work. The browser only noticed the CSS file if it completely replaced <link type="text/css" rel="stylesheet" href="res-min/themes/default.css">.

// So, in a worst-case scenario, you can just copy that CSS file, add your changes to it, host it on Github and do the replacement with a UserCustom extension (https://github.com/benweet/stackedit/wiki/UserCustom-extensions).